Room-temperature ionic liquids (ILs) have been primarily used as “green” solvents in chemical synthesis, electrochemistry, and other applications due to their ultra-low vapor pressure, non-flammability, and high thermal stability. Ionic liquids, as the name indicates, are composed solely of ions. The most commonly studied ionic liquids are those where the cation is one of the following: 1-alkyl-3-methylimidazolium, N-alkylpyridinium, tetraalkylammonium and tetraalkylphosphonium. The organic cations, which are generally relatively large compared with simple inorganic cations, account for the low melting points of the salts. Anions range from simple inorganic anions to large complex anions. The synthesis process is environmentally friendly and does not involve high pressures (usually ambient air) or high temperatures (usually 60-80° C.).
Ionic liquids have other possible features that make them attractive for tribological applications, including negligible volatility, non-flammability, high thermal stability, and better intrinsic performance. These characteristics may avoid the need to add expensive additives to facilitate lubrication, as in the case of conventional mineral-oil-based lubricants. Detergents may not be necessary because ionic liquids act as solvents, defoamers may not be necessary due to ionic liquids' ultra-low vapor pressure, anti-oxidants may not be necessary due to the high thermal stability of ionic liquids, and anti-wear additives may not be necessary if ionic liquids form boundary lubricating films.
Limited studies have shown the potential for using ionic liquids as a new class of lubricants. Friction and wear reduction have been reported on metallic and ceramic surfaces lubricated by selected ionic liquids. It has been suggested that adding a few percent of ionic liquids could improve the lubricating behavior of base oils. Most literature studies have been limited to ionic liquids with imidazolium cations and BF4− and PF6− anions.
Ammonium salts of partial esters of phosphoric and thiophosphoric acids are commercially available as extreme pressure and antiwear additives for lubricants and are disclosed in Sieberth, U.S. Pat. No. 5,464,549 and Norman et al., U.S. Pat. No. 5,942,470. Other patents disclosing ammonium salts of other large anions for lubricants include Nebzydoski, U.S. Pat. No. 3,951,973, Baldwin et al., U.S. Pat. No. 4,115,286, and Wildersohn et al., U.S. Pat. No. 4,950,414 where the anions are trithiocyanurate, bis[(mercaptohydrocarbyl)ethylenedioxy]borates, and cyclophosphetane derivatives, respectively. However, it remains a goal to identify ionic liquids that display superior lubricating properties as the primary lubricant or as lubricant additives, particularly for use with difficult-to-lubricate metals, like aluminum, and for ceramics. Ionic lubricants with anions that permit superior thermal stabilility and low water solubility are also desirable for lubricants and lubricant additives.